I've heard that Sony's MagicGate Memory Sticks (the white ones)can cause copy-protection problems. However, other brands and Sony's more commonly available regular (non-MagicGate) Memory Sticks (purple colored) work fine. The MP3 player on the Clie may give you a little message complaining about playing non-commercial MP3s, but rest assured, it will then go ahead and play the tunes for you.